Production of sulphuric acid by the contact process



June 16, 1936 c. B. CLARK PRODUCTION OF SULPHURC ACID BY THECONTCT lPROCESS 2 Sheets-Sheet 1 Filed May 18, 1955 MGM,

kmxmm INVENTOR.

C//P/.a 5. C`Z /q/Q/f/ ATTORNEY.

C. B. CLARK June 16, 1936.

PRODUCTION OF SULPHURIC ACID BY THE CONTACT PROCESS Filed May 18, 1933 2 Sheets-Sheet 2 Patented `lune 16, 1936 AET PRODUCTION OF SULPHURIC ACID BY THE CONTACT PROCESS Cyril B. Clark, Scarsdale, N. Y., assignor to American Cynamid Company, New York,

6 Claims.

This inventionrelates to the production of contact sulphuric acid and more particularly to the production of sulphuric acid from a combined plant in which a portion of the sulphur-containing material which is used as a source of SO2 is a sulphuric acid sludge from the the sulphuric acid purification of hydro-carbonaceous material, such as for example, petroleum products.

The rening of petroleum products and similar hydro-carbonaceous substances generally requires the step of treating these substances with concentrated sulphuric acid. In this operation an acid sludge residue is obtained which has hitherto been of relatively little value and has resulted in serious problems in regard to its safe disposal.

Attempts have been made to dispose of this sludge by burning it but such a procedure is generally unsatisfactory due to the sticky, acid, Odorous character of the material which makes it difficult to handle, the corrosive action of the acid compounds on the burner equipment and the steam boiler when it is used as a fuel for producing power, and due to the large volume of gases evolved containing oxides of sulphur and which may cause a nuisance in thickly populated communities and even in isolated locations may damage vegetation. Due to its obnoxious properties it cannot be dumped into streams. In many cases practice has, therefore, been to transport it to some out of the way spot and deposit it in some depression far from localities Where it can do no harm. Therefore the disposal of acid sludge has presented a serious problem to oil reineries.

Several processes have been proposed in the past for utilizing acid sludges as sources of SO2. These processes in general consist in decomposing the acid sludge by means of heat with concomitant reduction of a greater or lesser proportion of the S04 compounds by the hydro-carbonaceous material to form a gas containing sulphur dioxide and the utilization of this sulphur dioxide gas for producing sulphuric acid. As the demand in oil refineries is usually for strong acid, the Contact sulphuric acid process, which produces this grade of acid, is particularly applicable from the oil refiners standpoint. Until recently, none of the processes proposed have proven commercially successful.

The process described by I-Iechenbleikner, in U. S. application Serial No. 568,050, filed October l0, 1931, now Patent No. 1,953,225, dated April 3, 1934; is the first to achieve notable commercial success. In this process the acid sludge is heated in rotary kilns by direct contact with hot combustion gases which may be produced either from fuel oil material or if desired, from brimstone or sulphur bearing materials. It has also been recently proposed to thermally decompose certain types of acid sludge in externally red retorts with violent agitation, while keeping the sludge liquid by diluting With petroleum fractions.

Further complications are encountered because the gas is cold which necessitates heat exchange equipment in the contact plant and the SO2 content may fluctuate Within fairly Wide limits since the disengagement of SO2 in the sludge still or kiln varies, particularly in batch decomposition processes. Even when a continuous process is employed such as described in the Hechenbleikner patent above referred to, the SO2 evolution may iiuctuate because of the non-uniform composition of the sludge treated.

It has been proposed to overcome the disadvantage due to the presence of hydrocarbons by completely removing the same. In my co-pending application Serial No. 670,123 filed May 9, 1933, now Patent 2,019,554, issued Nov. 5, 1935, I have described a process involving a partial removal of the hydrocarbons to the point Where the residue on burning will not give over 23-28 mg. of water per cu. ft. of gas. These methods require additional equipment Whether the removal of the hydrocarbon vapors is by refrigeration and solution in a scrubbing liquid or by more or less complete burning in a precombustion furnace. Even when the hydrocarbons are removed, completely or partially, so as to result in a gas which will not give trouble after conversion because of acid mist, the gas is relatively cold and the SO2 content may fluctuate within considerable limits.

The present invention avoids the disadvantages referred to above and further reduces the cost of producing sulphuric acid from acid sludge Where the demand for sulphuric acid and the supply of acid sludge are such that the present invention can be used. According to the present invention, instead of running a sulphuric acid contact plant on acid sludge SO2 alone, a plant is operated with a combination of two sources of SO2, one being acid sludge, the other either sulphur or sulphide ore. It is, of course, possible to practice the present invention with all three sources of SO2 gases going into a single conversion plant, but in normal cases there is no advantage in using both sulphur and sulphide ores together with acid sludge as SO2 sources.

In the operation of the present invention, the acid sludge is thermally decomposed by any suitable methods, for example, by the Hechenbleik- 5 ner process above referred to. The SO2 gases obtained are freed from most of the water and the condensable hydrocarbons and are then mixed with the gas stream from a sulphur burner or from an ore burner, if necessary, with suitable addition of fresh air. rI'he combined gases are then passed through a converter which may be operated with either a platinum or vanadium catalyst, butwhich preferably is operated with a vanadium catalyst, and the S03 formed is absorbed in absorption towers in the usual manner. By combining gases coming from the-acid sludge still or kiln with dry gases which are produced by a sulphur or ore burning system a mixed gas is obtained which can be so `adjusted by varying the relative amounts of SO2 from the acid Ysludge and SO2 from sulphur or ore so that the content of uncondensable hydrocarbons is such astopro-` duce an amount of water in the converter not exceeding 30 mg. per cu. ft. At the same time,

Ythe SO2 concentration is regulated, which is of great importance because the fluctuation of the SO2 content of thegas'from the acid sludge kiln is `smoothed out or equalized. This Aadjustment rof .SO2 concentration is eected in two ways. In the lrst place, the large amount of gases coming from the sulphur or pyrites burner, which .areof uniform SO2 content, act as a fly wheel,

making fluctuations of SO2 content in the gases `fromthe racid sludge kiln a much smaller percentage fluctuation in the total gas. In the case :of a.sulphur burner it is also easily'possible to vary the concentration of SO2 from the sulphur burner either by producing a gas stronger Ythan necessary and admitting varying quantitiesV of additional air or, although less effectively, by varying the concentration of SO2 gases delivered by the sulphur burner itself.

While it is a preferred embodiment of the present invention to utilize the SO2 gases from the acid sludge still without removal of the uncondensablehydrocarbons, it should be understood that the invention in its broader aspects is not limited thereto and if desired the hydrocarbon gasesrwhich are uncondensable may be partially or Wholly removed before mixing withv the gases from a Ypyrites or sulphur burner. Thisrmodication, of course, eliminates onefunction of the present invention, namely, the dilution rof the hydrocarbon gases to harmless concentrations without the use of specialhydrocarbon removing equipment. However, .the important functions of adjustment of SO2 and, as will be described in greater detail below, the vvadjustment of temperature, still remain, and areliable and economical utilization of acid sludge as a source of SO2 is realized.

While the removal, vpartly or completely, of uncondensable hydrocarbons from the SO2 stream from theacid sludge still will usually be unnecessary, it becomes of real importance when it is 'desired to produce high strength oleum. Even if all of the hydrocarbons are removed from the acid Ysludge SO2, this gas after passing through the condenser, is still saturated with water vapor under those conditions, and the drying tower acid absorbsthe water vapor from the saturated gas stream. This amount of water is so great that if the drying tower acid is circulated .as part of the acid absorption system it will greatly `limit high strength oleum which can be produced even though no further water is generated by combustion of hydrocarbons. When, however, the present process is used,par ticularly, with sulphur burning equipment, the

water introduced into the drying acid per unit of SO2 is relatively small and the SO2 from the sulphur burning equipment produces additional S03 which permits obtaining high strength oleum in greater quantity. Thus, when oleum is to be produced in large quantities the present process possesses the additional important advantage that the water balance is adjustable and a product, oleum, can be obtained in amounts which would be impossible with the ordinary system using only gases from the ordinary acid sludge distillation retort. This special modification of the present invention, is therefore, of considerable importance for installations where the proN .duction of oleum is-an important factor, and it is significant that this added flexibility is obtained without further complications and the vsystem can be operated. with all the important advantages of simplicity for the production of 98% 'acid during most of the time and yet on Yshort notice can deliver oleum when a Asudden demand for this product arises.

While the present invention is not `broadly concerned with the'source of additional SO2 gases particular advantages are obtained when a su phur burner is employed or where an ore burner is used in a process not requiring the cold scrubbing of the gases. The gases from a sulphur burner, are, of course, at very high temperature and require no purification with either platinum 9 or vanadium catalysts. TheSOz gases from the acid sludge still, are cold because they have passed through a water condenser and an acid rying tower. Also, the additional air which must be added at some point in the process is normally cold. This has necessitated rather elaborate heatexchangers when acid sludge is the only source of SO2 for a particular plant and since the heat exchangers for the most part, involved utilizing the heat of conversion of the S62 to vS703 in the converter, they were particularly open to corrosion difficulties if the water contenter hydrogen content of the gases should rise to the point where there is danger of mist formation. Also the varying concentrations of SO2 .given off by the acid sludge still rendered the temperature control in the converterv rather unsatisfactory. This modification Yof the present process utilizes the heat of a -sulphur burner to supply theV necessary heat to the .cold SO2 gases from the acid sludge still, without the use of a heat Vexchanger through which converted S03 passes. Thisisan important advantage of this particular modification of the present invention because, as will be. pointed out below, it renders rinvolves a purification system in which the gases are scrubbed and therefore, are reduced in temperature in the same way as the gases from an acid sludge still. It is however, possible to operate an ore burning system and even under certain circumstancesa smelter gas system without cold purification where an eiective dust illter is employed and a non-poisoning vanadium catalyst is used in the converters. The acid obtained from such system, of course, is contaminated 2,044,419 with arsenic or other volatile metallic poisons,

unless the ore happens to be free from such poisons as is the case with certain special pyrite ores. However, arsenic free acid is not necessary, for example, for certain fertilizer processes. In such cases the ore burning equipment may be combined with the acid sludge still and serve to supply the additional heat in the same manner as a sulphur burner and, of course, enjoy the same advantage.

Naturally it is possible to combine an acid sludge still, an ore burner and a sulphur burner into one process, the cold gases from the acid sludge system and from the ore burner system being heated up by the hot gases from the sulphur burner. Such a system While theoretically of interest is usually not warranted because of the additional equipment required. However, in certain cases it may be desirable and is, of course, included as one embodiment of the present invention.

When the uncondensable hydrocarbon gases in the acid sludges SO2 stream are not removed and where it is desired to operate the Whole system Without production of acid mist, the proportion of the total SO2 supplied from the acid sludge, is, of course, limited since the amount of SO2 supplied either by the burning of sulphur or sulphide ores must necessarily be suiiicient to dilute the hydrocarbons or other hydrogen containing gases in the acid sludge SO2 stream to a point Where the total amount of water generated in the converter will not exceed the upper limit of approximately mg. per cu. ft. This fact, of course, decreases the proportion of acid sludge which can be utilized and in some cases where a large supply of acid sludge is available and the market for acid is not suiiciently great to permit the combustion of suilcient sulphur or sulphide ore in order to dilute the hydrocarbons below the point where acid mist is formed, it is desirable to employ a modification of the present process in which the proportion of SO2 from the acid sludge is increased. This may be done by reducing the uncondensable hydrocarbon content as described in my copending application above referred to.

The invention has been generally described without reference to particular design of acid sludge decomposition equipment, sulphur or ore burning equipment, etc. It is an advantage of the present invention that in its broader aspects it is not limited to particular apparatus design and can utilize much standard equipment. However, it is desirable to operate the present process with an ecient acid sludge decomposition process and, therefore, the invention will be described in greater detail in conjunction with the drawings Which show the process in diagrammatic form, in combination with an acid sludge decomposition system such as that described in the Hechenbleikner patent above referred to.

In Fig. 1 is a diagrammatic elevation of a system combining an acid sludge still and sulphur burning equipment; and,-

Fig. 2, is a diagrammatic representation of a combined acid sludge decomposing kiln and ore burning system.

In the drawings, the same equipment is given the same reference numeral and it will be noted that the acid sludge decomposition equipment is the same in both figures.

In the modication shown in Fig. 1, fuel is burned in furnace l and the gases passed in counter-current to a body of acid sludge in a.

-oil boot. or seal I0.

rotary kiln or retort 2, the sludge entering through the sludge inlet 3 and passing through zones of increasing temperature until it is discharged as a granulatable fuel through the fuel outlet ll. The SO2 gases containing water, hydrocarbcns, nitrogen, etc., pass 01T thru pipe 5, into the water cooled condenser 6, into which cooling Water enters through pipe 'l and leaves through pipe 8. Water and heavier condensable hydrocarbons condense out in the condenser and form two layers, the water being removed through the water boot or seal 9 and the oil through the The partially dried gases then pass through a pressure relief valve Il, are admixed with air through the valved pipe l2 and enter the drying tower I3, where they are dried by a circulation of sulphuric acid. The dehydrated gas stream leaves the top of the drying tower and after passing through a meter Iii enters the suction of the blower 23, from which it is delivered to a pipe 24 leading into the converter 25. The pipe 24 is provided with two valves, 2S and 21.

Air is taken into the dryer l5 of the sulphur burning equipment, which dryer is usually a tower with sulphuric acid circulated by the acid pump i6. The dehydrated air is then forced into a sulphur burner il' by means of a blower i8. In the sulphur burner SO2 is generated in the concentration desired for producing suitable strength gas after mixing with the SO2 from the acid sludge still. The gases leaving the sulphur burner are of very high temperature and if the proportion of these gases used in the combined process is high, it may be necessary to 'u reduce their temperature somewhat. This can be effected in the cooler I9 which is usually in the form of a boiler and from which the waste heat is recovered in the form of steam. A bypass pipe 58 controlled by a valve 59 is provided so that a greater or less quantity of the hot gases may be by-passed around the cooler. When in normal operation a portion of the gases pass through the cooler, a very eiective regulation of temperature of the combined gases reaching the l converter can be effected by regulation of the valve 59 which Will determine the amount of cooling given to the gases. After passing through the cooler or by-pass pipe, the gases are run through a dust iilter 20 andthence into a pipe 2| which is controlled by the valve 22.

The pipe 2l joins pipe 2li between the valves 26 and 2l'. Thus, by a suitable adjustment of the Valves 22 and 26 the desired proportioning of the gases from the acid sludge still and the sulphur i.

burning equipment can be effected. The adjustment should be such that the desired concentration of SO2, oxygen and hydrogen containing substance, is effected, i. e. that the hydrogen content is reduced below 2 to 3 mg. per cu. ft. of mixed gas. The oxygen and SO2 concentrations are preferably adjusted for best operation of the converter which normally means a 7 to 8% SO2 content. The temperature which is influenced by the proportion of gases from the sulphur burner and the amount of cooling iny the boiler IQ is suiicient to assure ignition on entering the converter. For tray-type converters this is preferably about 380 to 400 C., and for internal heat exchange converters of the Jaeger type the entering temperature may advantageously be about 225 to` 250 C.

The SO2 in mixed gases in passing through the converter is transformed into S03 and after leaving the converter the gases are preferably cooled '.in .the SO3. cooler A.28 toiasuitable temperature ;for.absorption, for .example,;from 20 to.300 C. .They then pass into the absorbing tower .29, where acid is .circulated .by 4means of .the z acid pump 30, thru acid cooler 13|. .The exhaust gases pass off to the stack thru vthe valvedppe 32andacid :isdrawn .offfromfthe system through the'valved pipe 33, `dilute acid beingintroduced intoxthe `circuitthrough Athe valved pipe 34.

.,InFig. 2, theiacidsludgestill, .condenser and `pressure .relief valve is the same asin Fig. 1, .and corresponding jparts bear thesame reference characters. Instead of discharging from the pressure v.relief valve into the ,drying tower,.however, the. gases lpass directly .into the blower 23 yand are thenceforced,throughthe pipe 24 Yinto the drying tower I3, additional air being added `through the valved pipe l2. The flow vthrough the Ypipe 24 is controlled `by the Valve 26.

.Airis .sucked through .the .ore burner 35, .by meansrof the blower 68 and the SO2 gas vformed passes through the pipe v35, .dust lter 31, into the scrubbing tower 38. The scrubbing tower vis provided with :a circulation of dilute sulphuric acid by means of the acid pump 39. The ore gas being undriedand containing small amounts .of S03,v some acidmist is .formed which is not re- .moved .by the scrubber. This mist is taken out .by a Cottrellprecipitator to which the gas is passed from thescrubber 38. .If desired the precipitator may be replaced by a coke box. After leaving the Cottrell or .coke box the blower 66 sends the gases through the pipe 4I controlled by the valve 42, the gas stream joining the acid sludge SO2 gas in Apipe 24fjust beyond valve 26 and the combinedstreams, after the addition of air through the valved pipe I2, pass through the common drying tower I3.

The dehydrated mixed'gases which have been adjusted Afor SO2 concentration by .suitable .ad- `justmentof the valves'26 and 42, leave the dryer through the pipe `43, pass through the meter I4 andenter the top ofthe heat exchanger 44 .leaving the bottom of the heat exchanger through pipe 45 which leads to the converter 25. Inthe converter' the SO2 is transformed into A:E303 and the converted gases leave through the valved pipe 46 and are separated into two streams by a T 41, the two legs of which are controlled by valves 48and 49. rAportion of the gases passes through the pipe 50 into the heatLeXchanger 44. The other portion of the gases passes through the pipe 5l into the absorber 29. The converted gases after preheating theincoming gases in the heat Yex- .changer 44 leave through a valved pipe v52 which joinsthe pipe 5l just beyond the valve-49. Absorption takes place in the absorber 29 inthe same manner as described in Fig. 1.

Since the gases from the scrubber 38 are cold it would be impossible lto start vup the'system without some external 'preheating of the gases -entering the converter. This is provided'by a starting furnace y53 which connects to the pipe 50 through a pipe 54 controlledby Valve 55. The

' top of the heat exchanger is also provided `with Van exhaust pipe 56 controlled by a valve 51. In order to start up, valve 4B is closed and valves and 51 are opened. The starting furnace is lighted and hot combustion gases pass up through the heat exchanger until the temperature is suffr- Yciently high so that SO2 gases can be preheated to a temperature at which conversion will begin `in the converter. Then valves 55 and 51 are 'closed and valve-48 is opened and the heat exchanger operates thereafter by means V4of la Ypor- .sludges .to effectfsuicient dilutionunlessthe systemisito ing conversion.

A.tion .of 4the converted gases l;as fdescribed above.

`flhespecidcembodiments in .the .two figures of .the drawings are .typical arrangements illustrating the present invention. It.:should be under- .stood,;however,.that .the invention is not limited to the` details'` shown .and .other modifications may .be employed. .The drawings: are, .of -course, .dia-

.grammaticin natureiasthe invention is noticoncerned broadly with ,the particular mechanical design of the individual units of equipment. AAny suitable apparatus. can beused and the particular type chosen will depend .on .the .conditions of .the

,particular installation. .The capacities of the :two

systemswillalso vary depending on the amount of uncondensable hydrocarbons given off bythe sludge, whichin turn will var-y with Vdifferent Of :cour-se, ther greater the amountof uncondensable. hydro-carbons -.thelarger .must .be the sulphur burningor oreburning plant,.in1order operate on an acid mist producing basis.

The-present inventionhas been described generally. in conjunction With anacidsludge decomposition processusingordinary combustion gases and producing ,an -SOzigas` of moderate .strength .2

comparable to .that :obtained 'from a sulphur burner or-,frorn -suchptypes of ore burners as are capable of giving acomparatively strong gas. For certain purposes vit.may, however, be desirable to combine the acid sludge .SO2 withanSOz gas, for example, `an :SO2 gas from a smelter, from dilute` H28 ggases, or. from certain low grade ores, in which the .concentration of YSO2 lslower than 'that desiredfor.eiicientfoperation of Vthe converter and particularlyfor ecient heatregulation in the converter since .difficulties areY frequently encountered'with ea gas'which is very weak because. of insufficientA heat generation dur- The -present `process ymay be modified to overcome the disadvantages .of low strength smelterorother SO2 gases. Thisxmay be effected intwo ways; either the :SO2 from the acid sludge,lwhich in some cases Willxnotzbe sufcientlyfstrong'to effect much adjustment upward of the-smelter vcrore gas may .be :.employedrwith suicient hydrocarbon content to assure adequate heat development in the converter or a very strong lSO2 gasmay `be obtained from acid sludge by utilizing combustion gases from sulphur containing material in .order to heat the acid sludge in an internally fired kiln. .This modification of the acid :sludge process is described in the application of I. Hechenbleikner, Serial No. 575,550 filed November.17, 1931. This results, of course, in a very strongfSO'e gas and. can, therefore, serve to adjust the SO2 content of a smelter gas or ore burning gas which is too low.

.Another'modiedacid sludgeprocess producing strong'SOz gas isthat described in application of Rumple, VSerial .'No. '101,137 led December .6,` 1933. In thisprocess instead of using combustion gases as the 'heatingfmedium' for decomposing'the sludge in an internally-.fired kiln,fan SO2 gas is built up Aand .re-circulated in a continuous cycle through the retort and through'continuous or intermittent heat exchangersforfstoves which may be redfwith any suitable fuel. When SO2 is thus used a circulating gas stream is produced from which Water and oils or condensable hydrocarbons are-condensed out after leaving the kiln and the residual gas consists of some to 88% SO2 with the balance non-condensable hydrocarbon gases, carbon dioxide and the like. A' portion of .this gas stream, either before or after lpassing through the heat exchangers or stoves, is

continuously removed and passes to the converter after suitable dilution with air. As this is a very strong SO2 gas it may be used to fortify smelter or ore burning gases which are deficient in SO2. This modification of the invention is, of course, also included and is of great practical importance under certain particular conditions. It is a further illustration of the great flexibility of the combined process of the present invention which permits its utilization under the most varying conditions of operation.

The invention is not limited to any particular design of sulphuric acid converter and a tray type of converter may be used or one with an internal heat exchanger, for example, such as described in the patent to A. O. Jaeger, No. 1,660,511 dated February 28, 1929. It should be noted that the temperature to which incoming gases must be heated will be diiTerent for the two types of converters. In the oase of the tray type converter, the temperature must be high enough to permit immediate ignition. In the case of the converter with internal heat exchanger, the temperature should be such as to satisfactorily cool the catalyst layer. This will normally vary from about 200 C., up to 250 C. or even somewhat higher in the case of relatively dilute gases. In the claims where it is stated that the gases are heated to a temperature suitable for entering the converter, it should be understood that this means a temperature suitable for the particular type of converter used in any given installation.

I claim:

1. A method of producing sulphuric anhydride which comprises subjecting residual sludge from the sulphuric acid purification of hydro-carbonaceous material to thermal decomposition with the formation of a gas containing SO2, removing the major portion of free water vapor and substantially all of the condensable hydrocarbons from said gas, producing an SO2 gas by the combustion of sulphur containing materials included in the class consisting of sulphur and sulphide ores, drying the gas, admixing the two gas streams in such proportions as to produce an SO2 gas having an SO2 and oxygen content suitable for conversion by the contact sulphuric acid process and containing an amount of combustible hydrogen-containing substances suiciently small so that the hydrogen content on combustion will not yield more than 28 milligrams of water per cubic foot of gas and passing said combined gas over a sulphuric acid contact mass at reaction temperature to produce sulphuric anhydride.

2. A method of producing sulphuric anhydride which comprises subjecting an acid sludge from the sulphuric acid purication of petroleum products to thermal decomposition by direct contact of the sludge with combustion gases at temperatures at which the S04 compounds of the sludge are reduced to SO2, removing the SO2 gas from the sludge, removing the major portion of free water vapor from said gas while leaving an amount of hydrocarbon therein, such that the gas has combustible hydrogen content in excess of 3 mgms. per cu. ft., producing SO2 gas by the combustion or" sulphur containing materials included in the class consisting of sulphur and sulphide ores, drying the gas, admixing the two gas streams in such proportions as to produce an SO2 gas having an SO2 and oxygen content suitable for conversion by the contact sulphuric acid process, the hydrocarbon content being suiciently diluted so that on combustion the water produced does not exceed 28 milligrams per cubic foot and passing said combined gas over a sulphuric acid contact mass at reaction temperature to produce sulphuric anhydride.

3. A method of producing sulphuric anhydride which comprises subjecting acid sludge from the sulphuric acid purification of petroleum products to thermal decomposition in a retort, the sludge being directly contacted with hot combustion gases, removing SO2 containing gas from the retort, removing the major portion of free water vapor from said gas while leaving an amount of hydrocarbon therein, such that the gas has combustible hydrogen content in excess of 3 mgms. per cu. ft., producing an SO2 gas by the combustion of sulphur in a sulphur burner drying the gas, the concentration of SO2 in said gas being greater than that required for conversion, mixing the two gas streams and adding additional air to adjust the SO2 and oxygen content, the addition of air being varied in proportion to the variation of SO2 evolution :from the acid sludge decomposition still mixing the SO2 gases from the sulphur burner with the acid sludge SO2 gases in such proportion as to dilute the hydrocarbon content to a point at which on combustion the water produced does not exceed 28 milligrams per cubic foot and nally passing the combinedgases over a sulphuric acid contact mass at reaction temperature to produce sulphuric anhydride.

4. A method of producing sulphuric anhydride which comprises subjecting residual sludge from the sulphuric acid purification of hydrocarbonaceous material to thermal decomposition with the formation of a gas containing SO2, the decomposition being eiected in such a manner as l to produce a gas containing more SO2 than that corresponding to the normal strength for conversion, removing the major portion of free water vapor from said gas while leaving an amount of hydrocarbon therein, such that the gas has combustible hydrogen content in excess of 3 mgms. per cu. ft., producing a dilute SO2 gas by the combustion of sulphide ores, drying the gas, admixing the two gas streams in such proportion that the SO2 and oxygen content is approximately normal for the contact sulphuric acid process, the amount of sulphide SO2 being suiiicient to dilute the combustible hydrogen-containing substances to a point at which on combustion the water produced does not exceed 28 milligrams per cubic foot, and passing the mixed gas at reaction temperature over a sulphuric acid contact mass to produce sulphuric anhydride.

5. A method of producing oleum which comprises subjecting the residual sludge from a sulphuric acid purication of petroleum products to thermal decomposition with the formation of a gas containing SO2, removing the major portion of free water vapor from said gas by condensation and by drying in a sulphuric acid drying tower, producing an SO2 gas by the combustion of a sulphur containing material to form a very high strength SO2 gas, drying the gas, admixing said concentrated SO2 gas with the dehydrated SO2 gas from the thermal decomposition of the acid sludge, and adjusting the oxygen content of the mixture to the proper concentration for conversion, the amounts of SO2 from acid sludge and sulphur-containing substances being suflicient so that the remaining concentration of combustible hydrogen-containing substances in the mixed. gas is diluted to a point where on combustion they produce an amount of water not exceeding 28 mgms. per cu. ft. of mixed gas, passing the adjusted gas stream through a contact sulphuric final absorption system, the amount of SO2 from the combustion of sulphur containing'substances being suciently large in proportion to the SO2 fromv the acid `sludge decompositionso that the S03 formed ismoreathan. sufcient to reactwith all of the Water abstracted by -the portion of the drying toweracid-,which is used in the absorption system, whereby oleum is produced.

6. A-method according to claim 5 in which the.

second SO2 source is from the burning of sulphur.

CYRIL vB. CLARK. 

